Old South Church voted overwhelmingly on Sunday to let its trustees sell one of the church’s two copies of the 1640 Bay Psalm Book, the first book published in the British North American Colonies, a rare text that specialists say could fetch more than $10 million at auction.

The vote came after an impassioned but respectful discussion in which a few members entreated their fellow congregants not to part with such a treasure — at least not yet — but many more speakers declared that now was the time to solidify Old South’s finances and invest in its future.

The proceeds of any sale will help cover costly repairs to the historic church and, leaders said, set the congregation on firmer financial ground as it endeavors to expand its work as a major progressive Christian church in Boston.

“We went from being an iconic church to being a mission church — that’s what happened,” said member Bill Adams of Quincy, echoing the words of the church’s former moderator, Vard Johnson, to him that morning. “Now we have a job to do to get outside these walls and make a difference.”

Two hundred seventy-one members voted in favor and 34 were opposed, well beyond the necessary two-thirds threshold.

A second proposal to sell a collection of early American communion silver owned by the church also passed easily. The congregation amended both proposals to limit the timeframe for a sale to 10 years.

The votes came after nearly two hours of discussion in a special meeting held between the church’s two Sunday morning services.

“The members and leadership went through an amazingly open congregational process, and it was as beautiful as it was difficult,” said the Rev. Nancy Taylor, senior minister of Old South. “You can’t ask for more than people’s passion and love for church and fulfillment of mission.”

The psalm book, the work of several leading Puritan ministers who wanted a more literal — and in their eyes, more theologically correct — translation of the Book of Psalms from the Hebrew, quickly became the standard psalter used on Sunday mornings throughout the Massachusetts Bay Colony, said Ian Quinn, a music professor at Yale University.

Eleven copies have survived; all are owned by major institutions. No copy has been on the market since 1947.

Both of Old South’s copies are housed in the Boston Public Library, across the street from Old South. The silver is in the custody of Boston’s Museum of Fine Arts.

The vote in favor of selling the “historic assets,” as the church calls them, came despite the strong objection of the church historian, Jeff Makholm, and several of the church’s former lay leaders.

“I’m really severely disappointed with what went on today,” Makholm said afterward. “There’s a certain amount of fiscal irresponsibility that goes along with selling centuries-old treasures for current programs, no matter how good you think they are.”

But most of those who spoke at Sunday’s meeting, including a number of the church’s current leaders, made a strong case for a sale. They said the church had a primary responsibility to invest in its building so that it could keep its doors open seven days a week as a “sanctuary in the city,” continue ministering to the poor, and expand its membership.

Phil Stern, chairman of the board of trustees, said Old South is in a far stronger place than it has been in many years; both membership and annual giving have doubled since 2006. But he said the church needed at least $7 million in repairs, including a new heating system, updated electrical infrastructure and fire suppression improvements, and as well as storm windows and other fixes to improve the church’s energy efficiency.

Addressing both the repair backlog and conservative budgeting for future needs, he said, would cost at least $400,000 a year — 20 percent more than the congregation takes in.

Proceeds from the sale of the historic items would probably boost the church’s endowment enough to cover that cost and put the church on strong footing going forward, he said. Otherwise, the church would have to either spend a greater portion of its $18 million endowment than the current agreed-upon limit of 4 percent — or else cut programs and staff.

Paul Kuenstner, chairman of the church’s operations committee, told the congregation that in his professional work for a foundation, he has encountered scores of historic homes and churches that have foundered under the burden of maintaining their buildings.

“Old South doesn’t have to be one of those organizations,” he said. “We have the ability to stay out of harm’s way.”

Some members argued that delaying the vote for a few months would do no harm; perhaps, they said, this would allow time to develop a new strategy for raising money or lowering costs, or at least buy time to reach greater consensus.

But Bettina Blake argued that the church made a great sacrifice in the late 19th century by giving up its beloved meeting house downtown in order to expand its mission.

“We need to have the courage to seize the moment,” she said. “We need to do it now.”

Afterwards, Pam Roberts, another former moderator of Old South, was thrilled.

“It is said that Christianity is always one generation away from extinct,” she said. “I feel that keenly; I think our forebears felt that keenly. I don’t think we can know how they would have felt about this vote, but I’d like to think they’d have said to do whatever it takes to keep the church thriving.”

Makholm said it was not yet clear to him how the vote would affect the congregation; those opposed to the sale, he said, were still in a state of shock.

But Gloria Platt of the Fenway, who has belonged to Old South since 1969 and who voted against the sale, said she and other like-minded people would accept the decision of the majority and go on as before.

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